1. Field of the Invention
The present invention relates to an electromagnetic valve and a brake control system adopting the electromagnetic valve as a hydraulic pressure control valve for increasing or decreasing brake hydraulic pressure.
2. Description of Related Art
Conventionally, in a brake control device for carrying out traction control or turn trace control (vehicle yaw control), an SR valve which is an electromagnetic valve is provided as a hydraulic pressure control valve for adjusting wheel cylinder pressure by opening and closing a conduit from a master cylinder to a pump. As shown in FIG. 12, the SR valve closes the hydraulic path by a valve body 111 biased in an arrow mark A direction by a spring 112.
However, according to the SR valve having such a structure, in the case where a brake pedal 113 is depressed and hydraulic pressure (master cylinder pressure) on the side of a master cylinder 114 is increased, the master cylinder pressure acts to the valve body 111 so that the valve body 111 is moved in a valve closing direction (arrow mark A direction). Accordingly, even when electricity is supplied to a solenoid 115, attracting force in an arrow mark B direction caused by electromagnetic force of the solenoid 115 may become deficient and the SR valve may not be opened.
Accordingly, when the SR valve is used in, for example, a brake control device for carrying out power assist brake control (PAB control), that is, a brake control device for carrying out pressure increase control in which when the brake pedal 113 is depressed, a pump 116 is operated and the wheel cylinder pressure is increased more than normal to thereby enhance wheel braking force, pressure increase control may not be carried out preferably.
Although as a measure therefor, there is conceivable a method of enlarging the size (performance) of the solenoid, then, the SR valve becomes large-sized. Hence, for example, there has been proposed an SR valve using a main valve and an auxiliary valve.
Such an SR valve has an auxiliary valve comprising a magnetic body for alternatively opening and closing an auxiliary path by supplying electricity to a solenoid. When the auxiliary path is opened in accordance with operation of the auxiliary valve, pressure difference applied on a main valve comprising a nonmagnetic body is alleviated, thereby assisting the main valve to open a main path (refer to DE19529363).
However, according to such an SR valve having the main valve and the auxiliary valve, it is difficult to carry out precise control.
In recent years, there has been intensified a tendency of carrying out power assist brake control for promoting braking performance by increasing wheel cylinder pressure when a brake pedal is depressed in addition to, for example, normal brake operation, antiskid control, traction control and turn trace control (vehicle yaw control) in a single brake control device. Therefore, with the above-described conventional SR valve which only carries out simple opening and closing operation, it is difficult to preferably carry out various control while clearing problems of pedal feeling, operational sound and so on.
Further, even when a constitution capable of carrying out complicated operation is added to the SR valve, with the complicated structure, the device is large-sized and the cost is increased which is not preferable.
The present invention has been carried out in order to solve the above-mentioned problems and it is an object of the present invention to provide an electromagnetic valve capable of preferably carrying out various control and a brake control device adopting the electromagnetic valve.